This invention relates to electrical circuits for shaping and digitizing analog signals, and more particularly to a digitizer circuit and digitizing method for a bar code scanner device or similar optical scanner.
Bar code readers are disclosed in U.S. Pat. Nos. 4,387,297, 4,409,470, 4,251,798, and 4,760,248, for example, all assigned to Symbol Technologies, Inc. The bar code readers described in these patents, as well as other devices of this type that are commercially available, employ a photodetector to generate an analog electrical signal representing light reflected from a bar code symbol. The analog electrical signal is usually digitized, i.e., converted to a binary signal having two levels, one or zero, high or low. The binary signal is then decoded to attempt to recover the bar code pattern. The validity of the decoding step is of course dependent upon how faithfully the analog signal has been digitized, and upon the quality of the bar code itself and its background, as well as upon the optical system used to transmit a light beam to the bar code symbol and collect reflected light. The DC level of the analog signal can shift, and various false peaks or dips can appear in the signal due to many factors. In an attempt to account for these types of false indications, one approach has been to detect the derivative and second derivative of the analog signal, and use these values to detect peaks and transitions; a device using these features is disclosed in U.S. Pat. No. 4,000,397, where zero crossings of the second derivative are detected at selected gating times that occur whenever the first derivative exceeds a threshold level, where the threshold level is determined by adding the peak-to-peak levels of the analog signal to a noise measurement signal. Another approach is disclosed in U.S. Pat. No. 4,749,879, also using first and second derivatives or the analog signal and a zero crossing detector gated by threshold detectors. These prior circuits have not be able to recognize some valid bar code patterns, however.
One example of a bar code symbol that presents problems in digitizing is one produced by a dot matrix printer. Here the symbol is created by a pattern of dots, and the gaps between the dots can produce transitions resembling the transitions between bar and space. The signal produced by a symbol printed by a dot matrix printer must be distinguishable, however, from a signal produced by a high density bar code, such as Code-49. A high density bar code symbol may produce a signal from the photodetector that is difficult to distinguish from a signal produced by a low density bar code symbol created by a dot matrix printer; the derivatives of these signals are even more difficult to distinguish.
It is the principal object of this invention to provide an improved bar code scanner device, and particularly one able to more effectively and/or efficiently decode the bar code symbols being scanned. Another object is to provide an improved digitizer for bar code readers or the like. A further object is to provide a bar code scanner device that is versatile, accurate and efficient in operation. An additional object is to provide an improved bar code scanner device that is able to more rapidly adapt to variable conditions, with a minimum of intervention by the operator.